Pulps produced with alkaline pulping methods, such as the Kraft method, or produced with acid pulping methods such as the acid magnesium bi-sulfite method, or with methods which use organic dissolving agents such as methanol (ORGANOSOLV.TM., ORGANOCELL.TM., ALCELL.TM.), or with alkali pulping methods which use, in addition to aqueous alkali, sulfite, anthraquinone and/or other organic solvents such as methanol, e.g. the ASAM method (Alkali-Sulfite-Anthraquinone-Methanol) must be treated in at least one bleaching step after pulping to achieve high degrees of brightness.
The state of the art technology for the production of paper or products made from dissolving pulp is based on the use of bleached fibers containing small amounts of residual lignin. An almost completely lignin-free pulp with an .alpha.-cellulose content of 98% is required, for example, for dissolving pulps. The fiber must also be free of lignin for chemical pulps as well. The brightness requirements for paper made from recycled fibers are also continually increasing. Fibers primarily used for the production of newspaper such as ground wood, RMP (refiner mechanical pulp), TMP (thermo mechanical pulp), and CTMP (chemo-thermo mechanical pulp) are increasingly being bleached to higher brightnesses, not only with reducing bleaching agents such as hypochlorite (HClO) and dithionite (SO.sub.2 O.sub.4.sup.-2) but also with oxidizing bleaching agents containing oxygen such as hydrogen peroxide. Because bleaching is no longer conducted exclusively with elemental chlorine or chlorine containing chemicals for environmental and economic reasons, chlorine-free oxidizing compounds like oxygen, ozone or peroxo-chemicals like hydrogen peroxide or peracids and mixtures thereof are used more often.
These chlorine-free chemicals comprise mainly oxidizing bleaching chemicals like oxygen, ozone and peroxo-chemicals. Among the peroxo-chemicals, peroxides, especially hydrogen peroxide is well suited to bleach lignocellulosic fibers. However, sodium hydroxide, peracids like peracetic acid, performic acid or Caroic acid and salts thereof are also suited to increase pulp brightness. The increasing trend towards the TCF (total chlorine free) bleaching of all fibrous materials for the production of paper with oxygen, ozone and chemicals containing peroxo compounds necessitates increased efforts to more efficiently utilize and activate these chemicals in the bleaching liquor, thereby attaining higher consumption and higher brightness.
It is very difficult, however, to activate hydrogen peroxide during this procedure by adding more alkali or increasing the temperature. The higher amounts of alkali or the higher temperature can greatly effect the bleaching reaction, leading to a complete consumption of the peroxide in the alkali milieu which results in secondary yellowing. (H. Suss; H. Kruger and K. Schmidt, "Die optimale Bleiche von Holzstoffen und ihre Abwasserbelastung", Papier (34), (10), 1980, pg. 433-438). Thus it has been necessary to retain a certain residual amount of peroxide in the alkali fiber suspension after bleaching to avoid brightness reversion after final bleaching of the fibers.
Peroxide bleaching of fibrous materials used for the production of chemical and dissolving pulps has become a normal practice today. Almost all types of pulps can be bleached at least in single bleaching steps with an alkaline peroxide solution (P stage), often a P-stage is used for brightening the pulp to final brightness in the final bleaching step. Even during prebleaching, delignifying treatment with oxygen (alkaline oxygen stage), increased brightness is attained by adding hydrogen peroxide. Pulps finally bleached with hydrogen peroxide demonstrate, however, a decreased brightness reversion compared to pulps bleached with a CEDED-sequence by application of elemental chlorine (C), alkaline extractions (E) and chlorine dioxide (D).
For the most part, it is impossible to attain a pulp brightness above 80% ISO for softwood pulps produced with the alkaline sulfate method (also known as the Kraft method) by TCF bleaching without using ozone and higher dosages of hydrogen peroxide which is not economical. Lab studies have reported on multistage bleaching methods which, with the exclusive use of 7% of hydrogen peroxide, attained a brightness of 88% ISO. These studies are described in "The optimal conditions for P* hydrogen peroxide bleaching" by Desprez, J. J. Devenyns and N. A. Troughten Proc. Pulping Conf. San Diego 929-934 (1994). However, cost of bleaching chemicals is extremely high for this bleaching sequence.
In order to improve the effect of bleaching of peroxo-compounds, efforts have been made to stabilize said peroxo-compounds, i.e. to prevent decomposition of e.g. hydrogen peroxide in the bleaching liquor. Known additives e.g. for stabilizing hydrogen peroxide are sodium silicate, EDTA or DTPA.
Bleaching is usually conducted in several stages. Between these stages the pulp is washed on washing filters. Because of the presence of heavy metal ions in the pulp, which were incorporated into the wood during growth, a chelation should be conducted before peroxide bleaching to reduce catalytic decomposition of peroxide. Chelation is conducted in a separate process step at temperatures between 50-90.degree. C. and under slightly acidic reaction conditions, e.g. pH level between 4-6 with soluble chelating agents such as EDTA (Ethylene Diamine Tetraacetic Acid) or DTPA (Diethylene Triamine Pentaacetic Acid), followed by washing. It can also be conducted at acid pH levels between 2-4 with sulfuric acid and at higher temperatures. In the following washing step the acid must be completely removed.